It is a common practice to acidize subterranean formations in order to increase the permeability thereof. For example, in the petroleum industry, it is conventional to inject an acidizing fluid into a well in order to increase the permeability of a surrounding hydrocarbon-bearing formation and thus facilitate the flow of hydrocarbons into the well from the formation. Such acidizing techniques are generally referred to as “matrix acidizing” procedures.
In matrix acidizing, the acidizing fluid is passed into the formation from the well at a pressure below the breakdown pressure of the formation. In this case, increase in permeability is effected primarily by the chemical reaction of the acid within the formation with little or no permeability increase being due to mechanical disruptions within the formation as in fracturing.
However, one critical factor to the success of a matrix acidizing treatment is the adequate placement of the acid so that all productive regions are contacted by the acid. Since there is significant difference in reservoir permeability, the acid trends to flow primarily in the zone of high permeability leaving low permeability zones untreated. Thus the techniques of acid placement during matrix acidizing are very important. The more the common techniques for acid diversion include: mechanical zone isolation, ball sealers, particulate diverging agents, viscosified acids and foams. However, each of these techniques has advantages and limitations.
For example, the ball sealers method is a popular diversion method whereby ball sealers are added to the treatment fluids, allowing the ball sealers to fill perforations or regions of high permeability. The ball sealers are usually recovered once the injection is terminated and the wellbore pressure drops. Ball sealers are mostly effective in newer wells with limited number of perforations. However, in older wells with damaged perforations or with large perforation density, the effectiveness of ball sealers is dramatically reduced. Ball sealers also require smooth and symmetrical perforations or homogeneous formations for the ball to seat well and divert the fluid to other zones. Additionally, in instances where the perforations or the formation is irregular or non-homogenous, the balls have a lower probability of sealing properly. Most importantly, the success in ball sealers diversion depends strongly on the injection rate for the balls to seal the “high space regions,” as well as the settling velocity of the ball in the carrying fluid, and thus the operation requires an excess of ball sealers to be pumped with the fluid to overcome these limitations.
For particulate diverging agents, a relatively low-permeability filter cake is formed on the formation face to achieve diversion. The pressure drop through this filter cake increases the flow resistance and thus diverts the acid to the other parts of the formation. In order for this operation to succeed, the particulate diverging agents must form a low-permeability filter cake and must be easily removed after treatment. For permeabilities above a certain value, the divergent agents do not have a good performance because they cause a great invaded region, especially in regions of the reservoir where different permeabilities exist, which complicates both the diverting process and the clean-up once the operation is finished.
Acids viscosification is accomplished by adding polymers and optional cross-linking agents. The mechanism of diversion is viscous diversion, by which the increase in flow resistance in higher-permeability regions occurs due to the presence of a bank of viscous fluid. However, the radius of the penetration of the viscosified acid is limited by the injected volume.
Foamed acids can act as diverting agents due to the reduced mobility in the rock resulting from the presence of the liquid film that separates the bubbles from the carrying liquid. This gives the foam an overall lower mobility in rocks. However, the efficiency of foamed acids is lower in damaged rock zones as compared to lower-permeability zones due to the foam effective viscosity, i.e. mobility differs between layers as well as the propagation rate. Foam, like acids, also increases the resistance to flow into a given interval by reducing liquid mobility.
As such, foamed acids, acid viscosification, and diverging agents all require the introduction of additional substances into the borehole, which further increases costs and complexity.